Highly absorbent, crosslinked polymers have found wide use in a variety of applications, including sanitary goods, hygienic goods, water retaining agents, dehydrating agents, sludge coagulants, condensation preventing agents and release control agents for various chemicals. Water-absorbent polymers are available in a variety of chemical forms including substituted and unsubstituted natural and synthetic polymers such as hydrolysis products of starch-acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, polyvinyl alcohols, polyacrylonitrile, polyvinylpyrrolidones, sulfonated polystyrenes, hydrolized polyacrylamides and polyethylene oxide.
In addition, aqueous gels, formed from the highly-absorbent crosslinked polymers of the present invention, have shown unexpected utility in increasing the crop yield of germinated plants. Such results are more surprising considering that it is not necessary to incorporate primary plant nutrients, micronutrients, growth promoters or other agricultural and/or horticultural adjuvants into the gel to increase the crop yields of germinated plants. Unexpectedly, it also has been found that aqueous gels including a mixed salt of homopolymerized or copolymerized acrylic acid generate higher crop yields when fertilizers are excluded from the aqueous gels.
It also has been found that it is unnecessary to admix inert solid aggregates, such as sand, rock, woodflour or vermiculite, with the mixed salt of the polymerized acrylic acid in order to help support a recently-germinated or a transplanted plant. The aqueous gels formed from the mixed salt polyacrylates of the present invention are of sufficient strength and rigidity to hold the plants upright, and also allow the roots of the germinated plant to withdraw the necessary water from the aqueous gel to preserve plant life.
Water-absorbent polymers have been used both to preserve freshly cut ornamental plants and as a growth medium for seeds, seedlings and transplants. U.S. Pat. No. 2,971,292 discloses a number of gel-forming colloidal materials, including polyacrylic polymers, that preserve the life of freshly cut plants. However, these gel-forming colloidal materials require the use of plant nutrients and the use of an inert solid aggregate filler to free water from the gel and thus make the water available for plant uptake. As will be seen in the detailed description of the invention, the inclusion of plant nutrients and inert solid aggregates into gels made from the polymers of the present invention is unnecessary, and is potentially detrimental.
Other patents disclosing the use of water absorbent polymers for use in plant preservation or as a plant growth medium include: U.S. Pat. No. 4,124,748, wherein a crosslinked copolymer of a vinyl ester and an unsaturated carboxylic acid ester, neutralized with a potassium or ammonium alkali, is suggested as a seed culturing media for plants; U.S. Pat. No. 4,241,537, wherein a nonionic, monolithic, crosslinked polyurethane is used as a soil plug for growing plants; U.S. Pat. No. 4,559,074, wherein a substantially nonionic crosslinked polyacrylamide is used as an additive for a plant growth medium; U.S. Pat. No. 4,238,374, wherein a water-insoluble crosslinked polymer and inert aggregate particles are utilized to preserve floral arrangements; U.S. Pat. No. 4,320,040, wherein a polyvinyl alcohol and polymerized acrylic acid composition is used as a water-retaining agent for plants or soils; and U.S. Pat. No. 3,336,129, wherein an absorbent crosslinked polymer and sand or soil are admixed to form plant growth modifiers. Several other U.S. Patents disclose polymers used in plant growth media, including U.S. Pat. Nos. 3,373,009; 3,900,378; 3,973,355; 4,034,508; 3,831,317; 4,495,310; 4,439,552; and 4,329,436.
The methods and compositions disclosed in the prior art require or recommend the inclusion of fertilizers and/or solid aggregates into the gels formed from the water-absorbent polymer. In addition, several of the prior art methods are difficult or impractical to use because: the compositions are not readily dispersed in water; the polymer, such as a starch-acrylonitrile graft polymer, is expensive and difficult to make; the polymer is subject to hydrolysis or bacterial degradation unless parameters, such as pH, are carefully controlled; the physical parameters, such as pH, necessary to protect the integrity of the polymer may adversely affect certain plants; and the polymers produce a gel that does not readily surrender water to the plants.
Therefore, it would be extremely advantageous to provide a method of increasing the crop yield of germinated plants by utilizing an aqueous gel including an economical, easy-to-synthesize, readily dispersible, non-degrading, water-absorbent polymer. It also would be advantageous if the polymer produced gels of sufficient gel strengths to support the stems of the plant without the need of inert solid aggregates, yet still be able to release the necessary water to the plant on demand. Finally, it would be most advantageous, both with respect to economy and ease of gel information, if aqueous gels formed from the polymer could be used without the addition of fertilizers and the like while providing nutrients to the plant.
Any method utilizing a polymer having the above-described qualities to increase crop yield would enhance and broaden the use of water-absorbent polymers in the agricultural and horticultural areas. Preferably, any such method should utilize an economical, easy-to-manufacture polymer that possesses qualities necessary to support plant life and improve crop yields, and that can be used at low percentages.